For a bottoming cycle apparatus, such as an apparatus based on the Rankine cycle, system efficiency is related directly to the up-time, that is, the operational time during which recovery of waste heat occurs. Inactive periods are often due to poor quality heat being available (not enough waste heat) or due to component warm-up time (when boilers and expansion machines are warming up).
The invention proposes a solution to increase operational time by improving thermal management during periods of poor quality heat availability and to decrease the warm up time of the apparatus when returning to operation.
The invention is applicable to bottoming cycles such as the Rankine cycle, the Ericsson cycle and other waste heat recuperating cycles.
According to the invention, an expansion machine of a bottoming cycle apparatus is connected in a working fluid circuit to receive working fluid from a heat recovery heat exchanger, such as a boiler, vaporizer, or heat exchanger. The working fluid directed to an expansion machine is expanded in the expansion machine to generate usable work or energy. The expansion machine also includes a heating jacket that is connected to receive working fluid for the purpose of heating the expansion machine. A bypass valve controls whether the working fluid is directed to the expansion inlet or the heating jacket.
Control of the bypass valve is based on the temperature of the working fluid (which may be measured at the outlet of the boiler) and the temperature of the expander (which may be measured at a convenient location). The bypass valve may also be regulated based on other conditions such as, but not limited to, control of expansion machine rotational speed, working fluid temperature regulation, or expansion machine torque demand (such as a request to stop power generation during engine brake mode).
According to the invention, an expander may be a turbine machine, a piston machine, a scroll, a screw, or another device capable of extracting useful work by expanding a working fluid. A multistage expander arrangement may be used in an apparatus according to the invention, with bypass being selectively controlled for one or more stages.
According to the invention, the heating jacket may be in the form of a water jacket.